1. Field of the Invention
The present invention relates to a bistable semiconductor laser diode device. The bistable semiconductor laser diode device is used, for example, for an optical communication system in a local area network (LAN).
2. Description of the Related Arts
In general, a bistable semiconductor laser is constituted by a laminated structure of an N-side electrode, an N-type layer, an active layer, a P-type layer, and a P-side electrode. This laminated structure is horizontally divided into two regions, that is, the gain region and the saturable absorption region. The gain region has a laminated structure of an N-side electrode, an N-type layer, an active layer, a P-type layer, and a P-side electrode. The saturable absorption region has a laminated structure of an N-side electrode, an N-type layer, an active layer, and a P-type layer, but does not have a P-side electrode. In the gain region, electric current passing between the N-side and P-side electrodes causes a stimulated emission in the active layer so that a light gain is attained, while, in the saturable absorption region, no electric current passes through the N-type layer, the active layer, and the P-type layer so that no light gain is attained.
As to the characteristic of the carrier concentration in the active layer versus the gain, in the range where the carrier concentration in the active layer is low, the gain assumes a negative gain value revealing the absorption state, while in the range where the carrier concentration in the active layer is high, the gain assumes a positive value revealing the stimulated emission state to cause a laser oscillation so that the light amplification function is achieved.
In the saturable absorption region, as the intensity of the light increases, the degree of the absorption decreases, and the gain approaches from a negative value to the saturated value close to zero. However, the gain does not assume a positive value, since the saturable absorption region does not have any pumping mechanism.
The bistable semiconductor laser device has a hysteresis characteristic, and it is possible to carry out a resetting operation of the device either by electrical means or by optical means. It has been proposed that, as one of the optical resetting means, the bistable semiconductor laser device is brought to the beat oscillation state by using neighboring frequencies to attain a resetting.
For example, a reference can be made to K. Inoue and K. Oe; "Optically Triggered Off-Switching in a Bistable Laser Diode Using a Two-Electrode DFB-LD", Electronics Letters Vol. 24, No. 9, Apr. 28, 1988. In this optical resetting method, when two frequencies are very close to each other, the total light output is changed with the beat frequency of the two frequencies. If the light output becomes lower than a predetermined value, the carrier pumping in the saturable absorption region will be decreased to reduce the carrier concentration and to increase the light absorption. If the degree of the light absorption becomes higher than a predetermined value, it will become impossible to maintain the laser oscillation to cause the resetting of the device.
However, the above-mentioned prior art electrical or optical resetting method has problems that, in the case of the electrical resetting, the use of an electrical signal for control will cause electrical noises which disturb the neighboring circuits and, particularly in integrated circuit devices, will cause cross-talk between electrical wirings, and, in the case of the optical resetting using the beat frequency, there is a difficult requirement to make the motion of the carriers follow the period of the beat oscillation which requires the control of the wavelength of the irradiation light with quite a high resolution such as a wavelength difference of the order of 0.1 angstrom.
Also, the above-mentioned prior art optical resetting method has a problem that the frequency beat cannot be satisfactorily attained when a reset light is injected into the active layer of the laser along an axis different from the lasing light axis and accordingly the resetting cannot be attained.